In the related art, an auto document feeder (ADF) which transports documents automatically is provided in image reading apparatuses, such as a scanner, a copying machine, and a facsimile machine. In this ADF, documents are transported along the transport path from a paper feed tray toward a paper discharge tray by a transport mechanism. A reading device is disposed on the transport path to read an image of the document.
When documents are transported by the transport mechanism, a plurality of documents may be transported in an overlap state. That is, double feeding may occur. In order to detect double feeding of documents in the related art, for example, a double feed detection sensor using an ultrasonic wave is provided in the image reading apparatus. However, using such a dedicated sensor increases the cost of the image reading apparatus.
In addition, Patent Document 1 discloses an image reading apparatus in which such a dedicated sensor is not provided and which performs edge detection processing in an end portion of a document for the read document data and determines that documents are transported in an overlap state if the number of edges detected is 2 or more.    [Patent Document 1] JP-A-2005-5837